In general, it is known that, in devices having an image acquisition element, such as endoscope systems, the depth of field is narrowed as the number of pixels in the image acquisition element becomes higher. Specifically, in the image acquisition element, when the pixel pitch (the horizontal and vertical size of one pixel) is reduced in order to increase the number of pixels, the permissible circle of confusion is accordingly reduced, and thus the depth of field of an image-acquisition device is narrowed.
Thus, there is a known endoscope in which one optical image from an objective lens is divided into two optical images having different focus positions by a polarizing beam splitter, one optical image is made to pass via a λ/4 wavelength plate and a mirror, the other optical image is made to pass via a mirror, the optical images are imaged on an image acquisition element, and the two images are composited, thereby acquiring an image in which the depth is extended (for example, see PTL 1 and PTL 2).
More specifically, PTL 1 and PTL 2 describe that a polarizing beam splitter is used as an optical-path dividing element, a depolarizing plate or a λ/4 wavelength plate is provided to suppress a luminance loss, two images are imaged on one image acquisition element to achieve a reduction in size, and a difference between the two images is corrected when the two images are composited. In particular, PTL 1 describes that the depolarizing plate is disposed between the polarizing beam splitter and the objective lens to convert light in the polarized state entering the polarizing beam splitter into circularly polarized light, thus achieving uniform polarized-light separation.
Furthermore, when there is a difference in luminance between two images, that is, far and near images, to be composited, luminance unevenness is caused in a spatial direction or in a temporal direction in the image obtained after compositing; therefore, in order to resolve this, PTL 3 describes that a subject is subjected to matching in the temporal direction to smooth the image signal, thereby suppressing luminance unevenness in the composited image.